Magnetoresistive device and method having improved Barkhausen noise suppression

ABSTRACT

An improved magnetoresistive device and method for fabricating the same which results in improved Barkhausen noise suppression. A generally coplanar device is described having an MR structure conductive region longitudinally biased by opposing permanent magnet layers separated therefrom by a non-magnetic metal or dielectric separation layer. significant reduction of the demagnetization energy near the MR-to-permanent magnet junction is achieved, particularly in the use of an elliptically shaped conductive region and the resultant generally coplanar device is readily fabricated and reproducible using a self-aligning process. The longitudinal bias technique described can be used in conjunction with all known transverse bias techniques.

This is a continuation of application Ser. No. 07/975,479, filed Nov.12, 1992 and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to the field ofmagnetoresistive ("MR") devices and methods for fabricating the same.More particularly, the present invention relates to magnetoresistivedevices and processes for manufacturing the same for use as an MR sensorwhich exhibits improved Barkhausen noise suppression characteristics.

Magnetoresistive sensors or heads are known to be useful in reading datafrom a magnetic surface with a sensitivity exceeding that of inductiveor thin film heads. In operation, a MR sensor is used to detect magneticfield signal changes from a magnetic surface due to the fact that theresistance of the MR sensor changes as a function of the direction andamount of magnetic flux being sensed. It is also generally known thatfor an MR sensor to function effectively, it must be subjected to atransverse bias field to linearize its response. Various techniques foreffectuating such transverse biasing are known including current shuntbiasing and soft adjacent film biasing. The transverse bias field isapplied normal to the plane of the magnetic media and parallel to thesurface of the MR sensor.

It is also known that an MR sensor may be utilized in conjunction with alongitudinal bias field extending parallel to the surface of themagnetic media and parallel to the major axis of the MR sensor.Stabilization of MR sensors by means of a longitudinal bias field isnecessary for their application with high track density disk files inorder to suppress Barkhausen noise. Barkhausen noise results fromunstable magnetic properties such as multi-domain activities within theMR element.

With respect to the application of a longitudinal bias field for thesuppression of Barkhausen noise in an MR sensor, a number of patentshave issued in this area, primarily dealing with "exchange bias" throughuse of an antiferromagnet coupled in some manner to the MR device.Exemplary of these patents are U.S. Pat. No. 4,663,685 "MagnetoresistiveRead Transducer Having Patterned Longitudinal Bias" issued May 5, 1987;U.S. Pat. No. 4,713,708 "Magnetoresistive Read Transducer" issued Dec.15, 1987; U.S. Pat. No. 4,809,109 "Magnetoresistive Read Transducer andMethod for Making the Improved Transducer" issued Feb. 28, 1989 and U.S.Pat. No. 4,825,325 "Magnetoresistive Read Transducer Assembly" issuedApr. 25, 1989. With these exchange bias MR sensors, the materials incurrent use to form an antiferromagnet, such as manganese and itsalloys, are known to be highly reactive and have poor thermalcharacteristics.

In an attempt to solve the problems inherent with the use of anantiferromagnet to provide longitudinal bias, a number of patents andpublications have described the use of an MR sensor stabilized throughthe use of permanent magnet films. Exemplary of the techniques known arethose described in U.S. Pat. No. 4,639,806 "Thin Film Magnetic HeadHaving a Magnetized Ferromagnetic Film on the MR Element" issued Jan.27, 1987; Hunt, R. P. and Jaecklin, A. A., "Composite Films as aDomain-Wall Barrier", Journal of Applied Physics, volume 37, no. 3, Mar.1, 1966; European Patent Application No. 0,375,646 published Jun. 27,1990 and European Patent Application No. 0,422,806 published Apr. 17,1991. Some of the previous designs using permanent magnets forBarkhausen noise suppression by application of a longitudinal bias tothe MR sensor have been shown to be generally not suited for use withclosely coupled magnetic shielding layers. The remainder of thetechniques previously described are not readily implementable andreproducible.

SUMMARY OF THE INVENTION

The magnetoresistive device according to the present invention includesa magnetoresistive structure having oppositely disposed end portionstransverse to a major axis of the device. A pair of permanent magnetlayers is disposed adjacent to, and spaced apart from, the end portionsof the magnetoresistive structure in a generally coplanar relationship.A further embodiment of a magnetoresistive device in accordance with thepresent invention includes a magnetoresistive conductive region and aseparation layer adjoining opposite end portions of the conductiveregion. First and second permanent magnet regions are contiguous withthe separation layer at the end portions of the conductive region inorder to provide a longitudinal bias to the magnetoresistive conductiveregion by means of the first and second permanent magnet regions. Thelongitudinal bias technique disclosed can be utilized in conjunctionwith all known transverse bias techniques including current shunt,barber pole and self-biasing methods.

In accordance with the method disclosed herein, a magnetoresistivedevice may be manufactured by defining a magnetoresistive conductiveregion on a magnetoresistive structure and overlying opposing endportions of the conductive region with an adjoining separation layer.First and second permanent magnet regions contiguous with the separationlayer at the end portions of the conductive region are formed to providelongitudinal bias to the magnetoresistive conductive region. Inaccordance with an alternative process of the present invention, amagnetoresistive device may be formed by providing a magnetoresistivestructure and defining a conductive region on the structure. Themagnetoresistive structure is removed surrounding the conductive regionand a separation layer is formed on the exposed surfaces of themagnetoresistive conductive region. A permanent magnet layer is producedadjoining the separation layer and the separation and permanent magnetlayers which overlie the conductive region are removed in order that theremaining portions of the permanent magnet layer can provide alongitudinal bias to the conductive region.

In a more specific process according to the present invention amagnetoresistive device may be formed by producing a magnetoresistivestructure on a device substrate and defining a conductive region withinthe structure. The magnetoresistive structure surrounding the conductiveregion is removed leaving exposed opposing end portions. A separationlayer is then placed over the substrate and the magnetoresistiveconductive region. A permanent magnet region contiguous with theseparation layer is formed and the separation layer and permanent magnetregion is removed from the conductive region except at the exposed endportions thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of the invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a prior art MR head in whichlongitudinal bias to suppress Barkhausen noise is provided throughexchange bias utilizing an antiferromagnet;

FIG. 2 is another cross-sectional representation of a prior art MRsensor utilizing the exchange bias technique;

FIG. 3 illustrates another cross-sectional view of a prior art MR headincluding an antiferromagnetic layer and overlying capping layer;

FIG. 4 illustrates another cross-sectional view of a prior art MR readtransducer utilizing a hard magnetic material for providing longitudinalbias to the MR read transducer;

FIG. 5 is a cross-sectional view of yet another prior art MR readtransducer also illustrating the application of longitudinal bias to anMR read transducer through the use of a hard magnetic layer;

FIG. 6 is a partial isometric view of a magnetoresistive device inaccordance with the present invention illustrating an exaggeratedseparation between the magnetoresistive conductive region and theadjoining permanent magnet regions for providing longitudinal bias tothe conductive region for Barkhausen noise suppression (BNS);

FIG. 7 is a partial cross-sectional view of the desired separationmaintained between a permanent magnet region and a magnetoresistivestructure by means of a separation layer;

FIG. 8 is a graphic representation of desired permanent magnet (PM) tomagnetoresistive structure (MRS) separation in angstroms sufficient toprovide adequate Barkhausen noise suppression;

FIG. 9 is a partial isometric view of an alternative embodiment of amagnetoresistive device in accordance with the present invention showingconductive leads contacting the permanent magnet regions when anon-magnetic metal separation layer between the magnetoresistiveconductive region and the permanent magnet regions is utilized;

FIG. 10 is another partial, cut-away cross-sectional view of amagnetoresistive device in accordance with the present invention furtherillustrating the separation layer between the magnetoresistive structureand the adjoining permanent magnet region as may be utilized in theembodiments shown in FIGS. 9 and 11;

FIG. 11 is a preferred embodiment of the present invention illustratinga magnetoresistive device having an elliptical conductive regionproviding significant advantages in lowering the demagnetization energydensity near the magnetoresistive conductive region-to-permanent magnetregion junction; and

FIGS. 12a-12h are simplified cross sectional views illustrative of aself-aligned process flow for manufacturing a magnetoresistive device inaccordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, a prior art MR head 20 such as described inU.S. Pat. No. 4,713,708 is shown. MR head 20 includes an active region22 in conjunction with passive end regions 24. A non-magnetic spacerlayer 28 overlies the MR layer 26 in the active region 22 but not withinthe passive end regions 24. An antiferromagnetic layer 32 overlies thesoft magnetic film 30 in the passive end regions and conductor leads 34overlie the antiferromagnetic layer 32. Utilizing this technique,longitudinal bias is achieved by not having within the passive endregions 24, a nonmagnetic spacer layer 28 interposed between the MRlayer 26 and the soft magnetic film 30. In this manner, an exchangebiasing effect propagates from the longitudinal biasing layer throughthe MR layer 26 to the soft magnetic film 30. Through magnetostatic andexchange coupling along MR layer 26, a single domain state is induced inactive region 22 through the single domain states of passive end regions24.

With reference additionally to FIG. 2, another prior art MR sensor suchas described in U.S. Pat. No. 4,809,109 is shown. MR sensor 40 includesa layer of ferromagnetic material 42 such as NiFe. A non-magnetic spacer46 separates ferromagnetic material 42 from a soft magnetic film layer44. Antiferromagnetic material 48 overlies ferromagnetic material 42 asshown. Antiferromagnetic material 48 is described as FeMn. A pair ofleads 50 is attached to MR sensor 40 by contacting antiferromagneticmaterial 48 as shown. The antiferromagnetic material 48 providesexchange bias to MR sensor 40 as previously described.

Referring also now to FIG. 3, a prior art MR head 60 such as describedin U.S. Pat. No. 4,825,325 is shown. MR head 60 comprises an NiFe MRlayer 62. Substrate 64 has formed thereon a soft magnetic material 66and a nonmagnetic spacer layer 68 underlying the MR layer 62. Anantiferromagnetic layer 70 on the order of less than 25 to 50 angstroms,overlies the MR layer 62. A capping layer 72 overlies theantiferromagnetic layer to prevent oxidation damage due to the highlyreactive nature of the ferro-manganese antiferromagnetic layer 70.Capping layer 72 is described as a stable dielectric.

In each of the previously described prior art embodiments utilizing"exchange bias" techniques through the use of an antiferromagnet,significant problems with poor thermal characteristics and the highlyreactive nature of the antiferromagnetic materials are encountered.

With reference to FIG. 4, a prior art MR read transducer 80 such asdescribed in European Patent Application No. 0,375,646 is shown. MR readtransducer 80 includes an MR layer 86 having passive end regions 84separated by an active region 82. Longitudinal bias to MR readtransducer 80 is provided by means of hard magnetic material 92 in thepassive end regions 84 only. Hard magnetic material 92 is verticallyseparated from MR layer 86 by non-magnetic separation layer 94 as shown.Another non-magnetic spacing layer 90 separates MR layer 86 from softmagnetic layer 88. Longitudinal bias of the MR layer 86 in the activeregion 82 is provided by means of the hard magnetic material 92 locatedover the passive end regions 84. Conductor leads 96 contact hardmagnetic material 92 in passive end regions 84. The stacked nature ofthe resultant device is less satisfactory than one which is generallycoplanar.

Referring now to FIG. 5, a prior art MR read transducer 100 inaccordance with the description of European Patent Application 0,422,806is shown. MR read transducer 100 includes an active region 102 andpassive end regions 104. MR layer 106 in active region 102 islongitudinally biased by means of hard magnetic layers 108 in passiveend regions 104. MR layer 106 is in direct contact magnetically andelectrically with hard magnetic layers 108 through abutting junctions110. As described in the patent application, the profile of the abuttingjunction includes two overlapping tapers which are a trade-off betweenelectrical and magnetic reliability.

With reference to FIG. 6, a simplified view of a magnetoresistive device120 in accordance with the present invention is shown. Magnetoresistivedevice 120 includes a central conductive region 122 and opposingpermanent magnet regions 124. Conductive region 122 may comprise amagnetoresistive structure 126 (MRS) which, in a preferred embodiment,may include the tri-layer structure illustrated additionally in FIG. 7.Magnetoresistive structure 126 may, therefore, include an MR layer 138(for example, 200-500 angstroms of NiFe) and an underlying soft magneticlayer 140 which may comprise 200-500 angstroms of NiFeMo. Soft magneticlayer 140 may also be referred to as soft adjacent layer (SAL). Itshould be noted that in certain applications, it may be desirable thatsoft magnetic layer 140 overlie MR layer 138. Magnetic spacing layer(MSL) 142 may be formed of 100-250 angstroms of tantalum.

Permanent magnet regions 124 are formed of a permanent magnet layer 128which, in a preferred embodiment, may comprise CoPt or other cobaltalloys such as CoCr. A pair of conductive leads 130, which may compriseAu, contact magnetoresistive device 120 at conductive region 122although electrical contact may be made to permanent magnet regions 124when a non-magnetic metal is utilized for separation layer 136 in lieuof a dielectric material.

Referring additionally to FIG. 8, separation between permanent magnet(PM) regions 124 and the magnetoresistive structure (MRS) 126 is shownto be most desirable in the region of 50 to 250 angstroms. Separation onthe order of 400 angstroms or less for shielded MR heads is consideredto provide adequate Barkhausen noise suppression (BNS) by providinglongitudinal bias to conductive region 122 of magnetoresistive device120 to induce a single domain state therein. Separation betweenconductive region 122 and permanent magnet regions 124 may be maintainedby means of separation layer 136. Separation layer 136 may be anonmagnetic metal such as chromium or, alternatively, a dielectricmaterial such as aluminum oxide sufficient for maintaining the desiredseparation between conductive region 122 and permanent magnet regions124.

Referring additionally now to FIG. 9, an alternative embodiment of amagnetoresistive device 150 in accordance with the present invention isshown. Magnetoresistive device 150 comprises a conductive region 152separated from adjoining permanent magnet regions 154. Conductive region152 may comprise MR structure 156 as further illustrated incross-sectional detail in FIG. 10. As with the previously describedembodiment, MR structure 156 may comprise a trilayer structure includingMR layer 164 and underlying soft magnetic layer 166. Magnetic spacinglayer 168 is interposed between MR layer 164 and soft magnetic layer166. Permanent magnet regions 154 are made up of a permanent magnetlayer 158 and are separated from conductive region 152 by means ofseparation layer 162. In the embodiment of FIG. 9, conductive leads 160contact magnetoresistive device 150 at permanent magnet regions 154.This is possible through the use of a nonmagnetic metal such as chromiumfor separation layer 162 in lieu of a dielectric.

The magnetoresistive device 150 conductive region 152 is, preferably, aslong as the desired track width dimension of the device. Use of anonmagnetic metal separation layer 162 allows conductive leads 160 tocontact permanent magnet regions 154 obviating the undesirable currentshunting effects which are normally encountered when electrical contactis made directly to conductive region 152.

Referring additionally now to FIG. 11, a preferred embodiment formagnetoresistive device 170 in accordance with the present invention isshown. In this embodiment, conductive region 172 is formed into anelliptical shape. Permanent magnet regions 174 remain separated fromconductive region 172 in the manner aforedescribed and shown in FIG. 10.The use of an elliptical shape for conductive region 172 providessignificant advantages in lowering the demagnetization energy densitynear the conductive region 172 to permanent magnet regions 174 junction.Similar advantages utilizing variations in the conductive region 172shape technique illustrated in FIG. 11 over the orthogonal boundarieshereinbefore described may be inferred and the use of other conicsection shapes, curvilinear spacing regions or non-orthogonal boundariesbetween conductive region 172 and permanent magnet regions 174 may beemployed.

Elliptical end portions 180 of conductive region 172 may be contacted byconductive leads 182, for example substantially as shown in phantomlines.

Referring additionally now to FIG. 12, a representative process flow formanufacturing a magnetoresistive device in accordance with the presentinvention is shown. MR structure 200, which may be a trilayer structureas previously described, is patterned with photoresist 202 to define anMR conductive region 204. Portions of MR structure 200 not covered byphotoresist 202 are etched away as shown in FIG. 12c. Separation layer206 is deposited on the remaining substrate end structure such that itadjoins the end portions of MR structure 200 at conductive region 204 toproduce substantially the structure shown previously in FIGS. 7 and 10.Separation layer 206 also overlies photoresist 202 as shown. Separationlayer 206 may comprise a non-magnetic metal 208 or, alternatively, adielectric material such as aluminum oxide. Permanent magnet regions 210are formed on top of separation layer 206 as shown in FIG. 12e includingthose portions overlying photoresist 202. Permanent magnet regions 210may be deposited isotropically and later set in orientation along themajor axis of the magnetoresistive device by the application of asuitable magnetic field following the final processing steps.Alternatively, permanent magnet regions 210 may be depositedanisotropically. An additional photoresist process step as shown in FIG.12f is employed to pattern photoresist 212 over the combined structurepreparatory to an ion milling step as shown in FIG. 12g to etch away theremainder of the permanent magnet regions 210 and separation layer 206to leave device ends 214. As a final processing step, the photoresist isremoved from the device leaving a magnetoresistive device in accordancewith the present invention as shown in FIG. 12h, having amagnetoresistive active region separated from coplanar permanent magnetregions for providing longitudinal bias thereto.

In the embodiments of the present invention, the matching of themagnetic fluxes of the composite of the MR film and permanent magnetfilms form a substantially continuous magnetic body with very lowdemagnetization energy in the vicinity of the MR-permanent magnetjunction. This is especially true in the utilization of an ellipticallyshaped magnetoresistive conductive region. The coercivity of the MRsensor is then very low on average but the coercivity of the permanentmagnet portion of the magnet body is very high. Therefore, the compositefilm group is not switched during normal operation of the device. Thetight magnetostatic coupling results in a net easy axis field inside theMR sensor which maintains the device in a single domain state and, thus,suppresses Barkhausen noise.

The magnetoresistive device and method of the present invention isreadily utilized with closely spaced ferromagnetic shields, which canpresent gap lengths (MRS to shield spacing) of approximately 1,000 to4,000 angstroms. Desired permanent magnet region to MR conductive regionspacing is most advantageous in the range of an order of magnitude lessthan the gap length. Therefore, a 2,000 angstrom gap length wouldindicate a 200 angstrom permanent magnet to MR conductive regionspacing.

While there have been described above the principles of the presentinvention in conjunction with specific apparatus and processing steps,it is to be clearly understood that this description is made only by wayof example and not as a limitation to the scope of the invention.

What is claimed is:
 1. A magnetoresistive device comprising:amagnetoresistive structure presenting first and second oppositelydisposed end portions transverse to a major axis thereof, saidmagnetoresistive structure comprising a magnetoresistive layer and agenerally coextensive soft magnetic layer disposed in an underlyingrelationship thereto; and first and second permanent magnet layersdisposed adjacent to, but spaced apart from, said first and second endportions of said magnetoresistive structure in a generally co-planarrelationship thereto, said first and second permanent magnet layers forproviding a longitudinal bias to said magnetoresistive structure whilebeing physically separated therefrom by a separation layer.
 2. Thedevice of claim 1 further comprising a magnetic spacing layer interposedbetween said magnetoresistive layer and said soft magnetic layer.
 3. Thedevice of claim 2 wherein said magnetic spacing layer comprisestantalum.
 4. The device of claim 1 wherein said magnetoresistive layercomprises NiFe.
 5. The device of claim 1 wherein said soft magneticlayer comprises NiFeMo.
 6. The device of claim 1 further comprisingfirst and second electrodes for providing electrical contact to saiddevice.
 7. The device of claim 6 wherein said first and secondelectrodes comprise Au.
 8. The device of claim 6 wherein said first andsecond electrodes contact said magnetoresistive structure.
 9. The deviceof claim 6 wherein said first and second electrodes contact said firstand second permanent magnet layers respectively.
 10. The device of claim1 wherein separation between said first and second permanent magnetlayers and said first and second end portions of said magnetoresistivestructure is less than 400 angstroms.
 11. The device of claim 1 whereinsaid first and second end portions of said magnetoresistive structureare orthogonal to said major axis.
 12. The device of claim 1 whereinsaid first and second end portions of said magnetoresistive structureare generally defined by an ellipse having its foci generally along saidmajor axis of said magnetoresistive structure.
 13. The device of claim 1wherein said first and second end portions of said magnetoresistivestructure are curvilinearly disposed within a plane generally defined bysaid magnetoresistive structure.
 14. The device of claim 1 wherein saidfirst and second end portions of said magnetoresistive structure arenon-orthogonal to said major axis.
 15. The device of claim 1 whereinseparation between said first and second permanent magnet layers andsaid first and second end portions of said magnetoresistive structure isdefined by a nonmagnetic metal layer.
 16. The device of claim 15 whereinsaid nonmagnetic metal layer is chromium.
 17. The device of claim 1wherein separation between said first and second permanent magnet layersand said first and second end portions of said magnetoresistivestructure is defined by a dielectric material.
 18. The device of claim 1wherein said permanent magnet layers comprise CoPt.
 19. Amagnetoresistive device comprising:a magnetoresistive structurecomprising a magnetoresistive layer and a generally coextensive softmagnetic layer disposed in an underlying relationship thereto; first andsecond permanent magnet layers adjacent to, but separated apart fromsaid magnetoresistive structure, said first and second permanent magnetlayers being generally co-planar with said magnetoresistive structureand providing a longitudinal bias thereto while being physicallyseparated therefrom by a separation layer.
 20. The device of claim 19comprising a separation between said first and second permanent magnetlayers and said magnetoresistive structure of less than 400 angstroms.21. The device of claim 20 wherein said separation is defined by anon-magnetic metal.
 22. The device of claim 21 wherein said nonmagneticmetal is chromium.
 23. The device of claim 19 wherein said separation isdefined by a dielectric material.
 24. The device of claim 23 whereinsaid dielectric material comprises aluminum oxide.
 25. The device ofclaim 19 further comprising a magnetic spacing layer interposed betweensaid magnetoresistive layer and said soft magnetic layer.
 26. The deviceof claim 25 wherein said magnetic spacing layer comprises tantalum. 27.The device of claim 19 wherein said magnetoresistive layer comprisesNiFe.
 28. The device of claim 19 wherein said soft magnetic layercomprises NiFeMo.
 29. The device of claim 19 wherein said permanentmagnet layers comprise CoPt.
 30. The device of claim 19 furthercomprising first and second electrodes for providing electrical contactto said device.
 31. The device of claim 30 wherein said first and secondelectrodes comprise Au.
 32. The device of claim 30 wherein said firstand second electrodes contact said magnetoresistive structure.
 33. Thedevice of claim 30 wherein said first and second electrodes contact saidfirst and second permanent magnet layers respectively.
 34. The device ofclaim 19 wherein first and second end portions of said magnetoresistivestructure transverse to a major axis thereof are orthogonal to saidmajor axis.
 35. The device of claim 19 wherein first and second endportions of said magnetoresistive structure are generally defined by anellipse having its foci generally lying along a major axis of saidmagnetoresistive structure.
 36. The device of claim 19 wherein first andsecond end portions of said magnetoresistive structure are curvilinearlydisposed within a plane generally defined by said magnetoresistivestructure.
 37. The device of claim 19 wherein said first and second endportions of said magnetoresistive structure transverse to a major axisthereof are non-orthogonal to said major axis.
 38. A magnetoresistivedevice comprising:a magnetoresistive conductive region comprising amagnetoresistive structure including a magnetoresistive layer and agenerally coextensive soft magnetic layer disposed in an underlyingrelationship thereto; a separation layer adjoining opposing end portionsof said conductive region; and first and second permanent magnet regionscontiguous and generally co-planar with said separation layer at saidend portions of said conductive region whereby said first and secondpermanent magnet regions provide longitudinal bias to saidmagnetoresistive conductive region and said separation layer spaces saidfirst and second permanent magnet regions from said magnetoresistiveconductive region.
 39. The device of claim 38 wherein saidmagnetoresistive conductive region comprises NiFe.
 40. The device ofclaim 38 further comprising a magnetic spacing layer interposed betweensaid magnetoresistive layer and said soft magnetic layer.
 41. The deviceof claim 38 wherein said magnetoresistive conductive region issubstantially elliptical in configuration.
 42. The device of claim 38wherein said separation layer comprises a non-magnetic metal.
 43. Thedevice of claim 42 wherein said nonmagnetic metal is chromium.
 44. Thedevice of claim 38 wherein said separation layer comprises a dielectricmaterial.
 45. The device of claim 38 wherein said first and secondmagnetic regions comprise CoPt.